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Antarctic freeze paved the way for baleen whales

By Michael Marshall

Antarctica’s icy blanket and huge, filter-feeding whales may be inextricably linked. It seems that when the South Pole first froze over, the expanding ice sheets may have completely transformed ecosystems in the surrounding oceans, even driving the evolution of baleen whales.

Up until around 34 million years ago, Antarctica was warm and lushly forested. Then, within just 200,000 years, glaciers formed and spread over the entire continent creating a vast frozen wasteland.

Sander Houben of Utrecht University and the Netherlands Organisation for Applied Scientific Research and his colleagues studied tiny fossilised organisms in sediment cores from the Antarctic seabed to find out how these changes on the continent changed the surrounding marine life.

Houben focused on single-celled organisms called dinoflagellates, which produce hard cysts that often fossilise. Before the ice emerged, the dinoflagellates were a mixed bunch – some species produced their own energy by photosynthesis, others fed on smaller organisms.

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But as soon as the ice began to spread, photosynthesising dinoflagellates disappeared, leaving only the predators. The same setup exists today&colon; dinoflagellates survive solely by feeding on other types of photosynthesising algae. “There is a black to white transition towards the modern-day state,” says Houben.

Shifting ice

Houben thinks sea ice was key. Nowadays, Antarctica is fringed by floating sheets of ice that grow in winter and shrink in summer. This means plankton can only thrive during a very narrow seasonal window, says Houben.

In summer, melting sea ice releases nutrients into the water. Combined with lengthening days, this triggers vast plankton blooms. The blooms are set upon by swarms of Antarctic krill, which in turn are eaten by many larger animals including baleen whales.

Houben thinks that the switch from year-round life to seasonal blooms, driven by expanding sea ice, forced the ecosystem to adapt. Dinoflagellates would not have been able to make a living by photosynthesising in the cold and dark winter conditions under the sea ice, but could thrive by eating the annual plankton blooms.

The consequences may have been felt far beyond the sea ice zone. A 2009 genetic study by Eske Willerslev of the University of Copenhagen in Denmark found that baleen whales split from toothed whales around 36 million years ago (Systematic Biology, DOI&colon; 10.1093/sysbio/syp060). That is suspiciously close to the onset of the Antarctic freeze.

Baleen whales feed by lunging into swarms of krill. They filter the seawater and trap the tiny krill using the sheets of baleen that line their mouths. Houben speculates that the shift to seasonal krill blooms may have driven their evolution. Filter feeding would have allowed them to take full advantage of the annual krill blooms.